Protein-protein interactions play an important role in biology, and modulating protein-protein interactions have been recognized as a successful strategy for drug discovery. One unmet need in the field is to be able to covalently trap two interacting proteins in vitro and identify covalently crosslinked sites on two proteins. Since only proximal residues of two proteins are crosslinked, identification of crosslinked sites allows mapping proximal protein-protein interfaces in solution. The resulting knowledge is useful in the design of agents (e.g., small molecules, peptides, antibodies, etc.) that modulate (e.g., inhibit or promote) such protein-protein interactions for therapeutic purposes.